Abstract.

Pinus cembra (L.) trees growing at an elevation of 1,800 m in the Mercantour Alps National Park in southern France have shown various degrees of damage (yellowing and needle loss) since the early 1980s. Two stands were selected, one populated by healthy and the other by damaged trees, and 1-year-old needles were analysed. The needles of the declining trees displayed chlorotic mottled patterns associated with a significantly lower level of total chlorophyll compared to healthy trees. Measurements of gas exchange showed a lower photosynthetic rate linked to a lower activity of ribulose bisphosphate carboxylase and a higher stomatal conductance in needles of declining trees compared to healthy trees. In contrast, phosphoenolpyruvate carboxylase activity was found to be higher in declining tree needles, reaching twice the level measured in healthy tree needles. A significant oxidative stress was also observed in the needles which presented a 68% higher malondialdehyde content than healthy tree needles. The atmospheric concentration of ozone was quite high (40 nl·l–1, monthly mean in summer) in both stands but preliminary results based on stomatal conductance and ozone concentration measurements suggested a higher ozone flux in needles of declining trees. Ozone is thus suspected as a contributing factor in the observed decline of Pinus cembra trees.